Honestly if tanjiro was an adult before this fight happens( for example,muzan escapes in this fight). Kanao can use the antidtote to heal tanjiro before he turns into demon. Tanjiro would know that one single scratch from muzan can kill him from that moment on, next time he meets muzan. He could kill muzan but it may end up same as what happened to yorrichi, muzan escapes using his getaway trick which is bad news again. Tanjiro had no choice actually in this fight, but to kill muzan in this fight using suns advantage against demons, before he becomes known as strongest to everyone including muzan.
